ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E
The operation of Green Idle for V+ under various load conditions
is illustrated in Figure 4. Under light load conditions, C1 is
maintained in a charged condition, and only a single oscillator
pulse is required to charge up C2. Under these conditions, V+
may actually overshoot 7.25 V** slightly.
OVERSHOOT
7.25V
1
V+
7V
2
OSC
LIGHT
LOAD
7.25V
1
V+
7V
2
OSC
MEDIUM
LOAD
7.25V
1
V+
7V
2
OSC
HEAVY
LOAD
NOTES
1FOR ADM3310E AND ADM3315E REPLACE WITH 6.5V.
2FOR ADM3310E AND ADM3315E REPLACE WITH 6.25V.
age; decreasing the load current decreases it. The ripple voltage
at light loads is naturally lower than that for high load currents.
Using Green Idle, the ripple voltage is determined by the high and
low thresholds of the Green Idle circuit. These are nominally 7 V*
and 7.25 V**, so the ripple is 250 mV under most load conditions.
With very light load conditions, there may be some overshoot
above 7.25 V**, so the ripple is slightly greater. Under heavy load
conditions where the output never reaches 7.25 V**, the Green Idle
circuit is inoperative and the ripple voltage is determined by the load
current, the same as in a normal charge pump.
What about Electromagnetic Compatibility?
Green Idle does not operate with a constant oscillator frequency.
As a result, the frequency and spectrum of the oscillator signal vary
with load. Any radiated and conducted emissions also vary accord-
ingly. Like other Analog Devices RS-232 transceiver products, the
ADM33xxE devices feature slew rate limiting and other techniques
to minimize radiated and conducted emissions.
Transmitter (Driver) Section
The drivers convert 3.3 V logic input levels into EIA-232 output
levels. With V
CC
= 3.0 V and driving an EIA-232 load, the output
voltage swing is typically
±
6.4 V (or
±
5.5 V for ADM3310E
and ADM3315E).
Unused inputs may be left unconnected, as an internal 400 kV
pull-up resistor pulls them high forcing the outputs into a low
state. The input pull-up resistors typically source 8 mA when
grounded, so unused inputs should either be connected to V
CC
or left unconnected in order to minimize power consumption.
Receiver Section
Figure 4. Operation of Green Idle under Various Load
Conditions
Under medium load conditions, it may take several cycles for C2
to charge up to 7.25 V**. The average frequency of the oscillator
is higher because there are more pulses in each burst and the
bursts of pulses are closer together and more frequent.
Under high load conditions, the oscillator is on continuously if
the charge pump output cannot reach 7.25 V**.
Green Idle Versus Shutdown
The receivers are inverting level shifters that accept RS-232 input
levels and translate them into 3.3 V logic output levels. The inputs
have internal 5 kW pull-down resistors (22 kW for the ADM3310E)
to ground and are also protected against overvoltages of up to
±30
V.
Unconnected inputs are pulled to 0 V by the internal 5 kW (or
22 kW for the ADM3315E) pull-down resistor. This, therefore,
results in a Logic 1 output level for unconnected inputs or for
inputs connected to GND.
The receivers have Schmitt trigger inputs with a hysteresis level
of 0.14 V. This ensures error-free reception for both noisy inputs
and for inputs with slow transition times.
ENABLE AND SHUTDOWN
Shutdown mode minimizes power consumption by shutting down
the charge pump altogether. In this mode, the switches in the
voltage tripler are configured so V+ is connected directly to V
CC
.
V– is zero because there is no charge pump operation to charge C5.
This means there is a delay when coming out of Shutdown mode
before V+ and V– achieve their normal operating voltages. Green
Idle maintains the transmitter supply voltages under transmitter
idle conditions so this delay does not occur.
Doesn’t it Increase Supply Voltage Ripple?
The ripple on the output voltage of a charge pump operating in
open-loop depends on three factors: the oscillator frequency, the
value of the reservoir capacitor, and the load current. The value of
the reservoir capacitor is fixed. Increasing the oscillator frequency
decreases the ripple voltage; decreasing the oscillator frequency
increases it. Increasing the load current increases the ripple volt-
The enable function is intended to facilitate data bus connections
where it is desirable to three-state the receiver outputs. In the
disabled mode, all receiver outputs are placed in a high imped-
ance state. The shutdown function is intended to shut the device
down, thereby minimizing the quiescent current. In shutdown,
all transmitters are disabled. All receivers are shut down, except
for receiver R3 (ADM3307E, ADM3312E, and ADM3315E),
receiver R5 (ADM3311E), and receivers R4 and R5 (ADM3310E).
Note that disabled transmitters are not three-stated in shutdown,
so it is not permitted to connect multiple (RS-232) driver outputs
together.
The shutdown feature is very useful in battery-operated systems
since it reduces the power consumption to 66 nW. During shut-
down, the charge pump is also disabled. When exiting shutdown,
the charge pump is restarted and it takes approximately 100
ms
for it to reach its steady state operating conditions.
NOTES
*For
ADM3310E and ADM3315E, replace with 6.25 V.
**For
ADM3310E and ADM3315E, replace with 6.5 V.
–12–
REV. G
AD [ ANALOG DEVICES ]
